Molybdenum oxide, particularly MoO.sub.3, is widely used as a raw material in the manufacture of stainless and low alloy steels, pure molybdenum metal, superalloys, catalysts, and specialty chemicals. Molybdenum oxide is commonly produced from molybdenum sulfides, particularly molybdenum concentrates, that are obtained by grinding copper or molybdenum ores and concentrating the sulfides contained therein.
To convert molybdenum sulfide to molybdenum oxide, molybdenum sulfide is typically air roasted in a multiple-hearth roaster according to the following overall reaction: EQU MoS.sub.2 +3.50.sub.2.fwdarw.MoO.sub.3 +2SO.sub.2.
In multiple-hearth roasters, a static bed of feed material containing the molybdenum sulfides is calcined by an oxidizing gas (air) at temperatures ranging from 550 to over 700.degree. C. To avoid severe fusion of the bed, operators periodically mix the bed manually to maintain bed porosity and permeability at desired levels.
In designing a more efficient roaster for molybdenum sulfide concentrates, there are a number of important considerations. By way of example, the roaster should be continuous and produce a uniform quality, low-sulfur molybdenum oxide product. To reduce the capital and operating costs of gas handling and acid plants, it is desirable to minimize the use of excess air and thereby produce a roaster off-gas containing a relatively high concentration of SO.sub.2. Second, the roaster should be capable of automated operation to provide reduced operating costs. In other words, the roaster design should allow automated process control at operating conditions that will allow long operating times without downtime for cleaning and maintenance. Third, the roaster should recover heat energy released during sulfide oxidation as steam for useful purposes. Fourth, the roaster should have relatively few moving parts to improve system reliability, simplify system operation and decrease downtime and maintenance costs. Fifth, the roaster should provide for substantially uniform distribution of heat of reaction throughout the bed. The existence of temperature gradients in the bed can create hot zones where high temperature can cause partial fusion and sintering of the bed and volatilization of molybdenum oxide. Sixth, the roaster should have little, if any, refractory lining. Refractory lining can cause molybdenum loss (via molybdenum penetration into the refractory lining) and product contamination. Seventh, the roaster should eliminate the formation of hard crusts of molybdates and oxides. Such crusts can erode moving parts (e.g., rabble arms and teeth in conventional multiple-hearth roasters) and increase operating costs through increased labor to clean the roaster. Eighth, the roaster off-gas should have little, if any, dust entrainment to minimize product loss and downstream gas cleaning costs. Ninth, the roaster should be capable of handling feed materials comprising impurities, such as calcium, copper, iron and rhenium without operational problems. Finally, the roaster should operate at a temperature low enough to retard the volatilization of molybdenum trioxide and later condensation of the molybdenum trioxide in cooler pipes. Such condensation can cause operational and maintenance problems and reductions in product yields.